Field of the invention

The present invention relates to a process of recovering acetic acid from vaporous aqueous streams by means of absorption .

Background of the invention

Several oxidative chemical conversion processes known in the art produce aqueous streams comprising acetic acid as a side product. For example, it is known to oxidatively dehydrogenate ethane to produce ethylene in an oxidative dehydrogenation (oxydehydrogenation; ODH) process. The ethylene thus produced may be further oxidized under the same conditions into acetic acid. Other examples include the dehydrogenation of alcohols, the oxidation of aldehydes and the conversion (e.g. fermentation, pyrolysis, liquefaction) of biomass .

In the above process as well as in other oxidative conversion process, the acetic acid thus produced is

generally considered as a waste product. Although the acetic acid could be condensed together with water from the reactor effluent as an aqueous acetic acid (ca. 10 wt%) stream, the low relative volatility of acetic acid to water renders ordinary distillative separation of acetic acid and water troublesome, as this would require very large condensate recycle and/or separation trains.

However, acetic acid is a valuable ingredient and building block for use in the chemical industry. Acetic acid is used in the production of cellulose acetate for

photographic film and polyvinyl acetate for wood glue, as well as synthetic fibers and fabrics. In households, diluted acetic acid is often used in descaling agents. In the food industry, acetic acid is an approved food additive for use as an acidity regulator and as a condiment. The global demand for acetic acid is around 6.5 million tonnes per year (Mt/a), of which approximately 1.5 million tonnes is met by

recycling; the remainder is manufactured from petrochemical feedstock. As a chemical reagent, biological sources of acetic acid are of interest, but generally uncompetitive.

It is an objective of the present invention to provide a technically advantageous, efficient and affordable process for recovering acetic acid from vaporous aqueous streams.

Summary of the invention

It was surprisingly found that the above-mentioned objective can be attained by means of an absorption process for recovering acetic acid from a mixture of acetic acid and water, wherein the absorbing solvent is characterized by a small Hansen solubility parameter distance R _a with respect to acetic acid, a relatively high 1-octanol/water partition coefficient logP ₀ w and a boiling point exceeding that of acetic acid.

Accordingly, in a first aspect the present invention pertains to a process for the recovery of a acetic acid from an aqueous stream, comprising

providing a vaporous aqueous stream comprising acetic acid,

contacting said vaporous aqueous stream comprising acetic acid with an absorbing solvent in an absorption unit, to produce a first stream comprising absorbing solvent and acetic acid and a second stream comprising water vapour,

feeding said first stream comprising absorbing solvent and acetic acid to a solvent recovery unit, to produce a third stream comprising acetic acid and a fourth stream comprising absorbing solvent,

and optionally recycling at least a portion of the fourth stream comprising absorbing solvent to the absorption unit,

wherein the absorbing solvent is an oxygen-containing organic compound having

(i) a Hansen solubility parameter distance R _a with respect to acetic acid as determined at 25 °C of 15 MPa ^{1 2} or less, preferably 12 MPa ^{1 2} or less, more preferably 10 MPa ^{1 2} or less;

(ii) a 1-octanol/water partition coefficient logP ₀ w as determined at 25 °C and pH 7 of at least 0, preferably at least 0.5, more preferably at least 1.0, even more preferably at least 1.5, yet even more preferably at least 2.0, most preferably at least 3.0; and

(iii) a boiling point at atmospheric pressure that is at least 5 °C higher, preferably at least 10 °C higher, more preferably at least 20 °C higher than the boiling point of acetic acid.

The invention further relates to a process for the recovery of acetic acid from a vaporous aqueous stream, comprising

providing a vaporous aqueous stream comprising acetic acid,

contacting said aqueous stream comprising acetic acid with an absorbing solvent in an absorption unit, to produce a first stream comprising absorbing solvent and acetic acid and a second stream comprising water vapour,

feeding said first stream comprising absorbing solvent and acetic acid to a solvent recovery unit, to produce a third stream comprising acetic acid and a fourth stream comprising absorbing solvent, and optionally recycling at least a portion of the fourth stream comprising absorbing solvent to the absorption unit ,

wherein the absorbing solvent is a cyclic or aromatic alcohol having 6 to 20 carbon atoms, a linear aliphatic alcohol having 6 to 14 carbon atoms or a branched aliphatic alcohol having 5 to 14 carbon atoms.

In another aspect, the invention relates to the use of an oxygen-containing organic compound having

(i) a Hansen solubility parameter distance R _a with respect to acetic acid as determined at 25 °C of 15 MPa ^{1 2} or less, preferably 12 MPa ^{1 2} or less, more preferably 10 MPa ^{1 2} or less;

(ii) a 1-octanol/water partition coefficient logP ₀ w as determined at 25 °C and pH 7 of at least 0, preferably at least 0.5, more preferably at least 1.0, even more preferably at least 1.5, yet even more preferably at least 2.0, most preferably at least 3.0; and

(iii) a boiling point of at least 125 °C, preferably at least 140 °C, more preferably at least 160 °C, even more preferably at least 180 °C, most preferably at least 200 °C at atmospheric pressure

as a solvent for absorbing acetic acid from a water- containing vapour stream. Brief description of the drawings

Figure 1 shows an embodiment of the present invention, wherein an aqueous vapour stream comprising acetic acid is contacted with an absorbing solvent in an absorption unit, to produce a top stream comprising water vapour and a bottom stream comprising absorbing solvent and acetic acid, and wherein said bottom stream comprising absorbing solvent and acetic acid is fed to a solvent recovery unit to produce a top stream comprising acetic acid and a bottom stream comprising absorbing solvent .

Detailed description of the invention

While the process of the present invention and the streams used in said process are described in terms of

"comprising", "containing" or "including" one or more various described steps and components, respectively, they can also "consist essentially of" or "consist of" said one or more various described steps and components, respectively.

The aqueous stream comprising acetic acid used as feed stream for the absorption process may be any stream

comprising at least 0.1, or at least 1 wt%, more preferably at least 3 wt%, even more preferably at least 5 wt%, yet even more preferably at least 10 wt% or 15 wt%, most preferably at least 20 wt% wt% of acetic acid. Typically, said aqueous stream comprising acetic acid originates from an oxidative chemical conversion process of ethane and/or ethylene, wherein the acetic acid is obtained as a side product. It is preferred that the aqueous feed stream of the absorption process comprises acetic acid in a concentration of at least 1 wt%, more preferably at least 3 wt%, even more preferably at least 5 wt%, yet even more preferably at least 10 wt%, most preferably at least 20 wt%.

A concentration step, for example of a dilute aqueous gaseous process effluent comprising acetic acid, may be applied prior to contacting the acetic acid with the

absorbing solvent in the absorption unit. Such concentration step may comprise any suitable method for removing excess water from an aqueous acetic acid stream, including reverse osmosis or carboxylic acid-selective pervaporation, liquid- liquid extraction or adsorption on a solid adsorbant. In one embodiment of the invention, a vaporous effluent comprising acetic acid is treated using carboxylic acid- selective pervaporation to produce a concentrated acetic acid/water vapour stream, which is subsequently separated using an absorption process as described herein. In another embodiment, a vaporous effluent comprising acetic acid is concentrated by condensation followed by liquid-liquid (L/L) extraction with a high-boiling solvent and distillation of the more concentrated acetic acid/water vapour stream from the high boiling solvent and, finally, separation using absorption from the vapour phase as described herein. In another embodiment, a vaporous effluent comprising acetic acid is concentrated by adsorption onto a solid, followed by desorption of a more concentrated acetic acid/water vapour stream subsequently separated using absorption from the vapour phase as described herein.

Typically, such a concentration step yields an aqueous feed stream comprising acetic acid in a concentration of at least 5 wt%, more preferably at least 10 wt%, even more preferably at least 15 wt%, most preferably at least 20 wt%.

The vaporous phase stream comprising water and acetic acid may be the effluent stream from a gas-phase (oxidative) conversion process of ethane and/or ethylene. By directly subjecting the vaporous effluent comprising acetic acid and water of such process to the absorption step, capital and operating expenditure on excessive condensation and reheating steps can be avoided.

In one embodiment, the aqueous stream comprising acetic acid originates from the oxidative dehydrogenation ("ODH") of ethane. This oxidative alkane dehydrogenation process typically produces a product stream comprising ethylene and carbon dioxide, as well as water and acetic acid. In another embodiment, the aqueous stream comprising acetic acid originates from the oxidation of ethylene. In the absorption process of the invention, the gaseous aqueous stream

comprising acetic acid is contacted with an absorbing solvent in a suitable absorption unit in order to separate the carbocyclic acid from water. Several absorption methods for vaporous streams are available in the art. For example, absorption may suitably be performed in an absorption column, typically a multi-stage countercurrent packed or tray absorption column having inlets for receiving a vaporous feed stream and for absorbing solvent, wherein absorbing solvent is continuously fed at an upper stage of the absorption column, and wherein the acetic acid is absorbed by the solvent via direct contact of the rising vapour stream and the falling solvent.

Generally, choice of absorbing solvent is of high importance in the absorption process, since suitable

absorbing solvents can decrease the solvent ratio and/or the liquid load of the absorption unit, thus rendering an easy and more economical implementation of the absorption set-up, such as an absorption column, in a process line-up.

The present inventors have now surprisingly found that certain oxygen-containing solvents being characterized by (i) a short Hansen solubility parameter distance R _a with respect to acetic acid, (ii) a partition logP ₀ w as determined at 25 °C and pH 7 of at least 0, and (iii) a boiling point at atmospheric pressure that is at least 5 °C higher, preferably at least 10 °C higher, more preferably at least 20 °C higher than the boiling point of acetic acid, are excellent

absorbing solvents for use in a process for recovering acetic acid from aqueous vapour streams comprising acetic acid.

Hansen solubility parameters (HSP) can be used as a means for predicting the likeliness of one compound (solvent) dissolving in another. More specifically, each compound is characterized by three Hansen parameters, each generally expressed in MPa ⁰ ' ⁵ : 5 _d , denoting the energy from dispersion forces between molecules; δ _ρ , denoting the energy from dipolar intermolecular forces between molecules; and 5 _h , denoting the energy from hydrogen bonds between molecules. The affinity between compounds can be described using a multidimensional vector that quantifies these solvent atomic and molecular interactions, as a Hansen solubility parameters (HSP) distance R _a which is defined in Equation (1) :

R _a ) ² = 4(5 _d2 - 5 _dl ) ² + (δ _ρ2 - δ _ρ1 ) ² + (5 _h2 - 5 _hl ) ² (1) wherein

R _a = distance in HSP space between compound 1 and compound 2 (MPa ^0-5 )

δ _< η, δ _ρ ι , 5 _h i = Hansen (or equivalent) parameter for compound 1 (in MPa ^0-5 )

5 _d 2, δ _ρ2 , 5h2 = Hansen (or equivalent) parameter for compound 2 (in MPa ^0-5 )

Thus, in the context of the present invention, the smaller the value for R _a for a given solvent calculated with respect to the acetic acid to be recovered (i.e., acetic acid being compound 1 and the solvent being compound 2, or vice versa), the higher the affinity of this solvent for acetic acid to be recovered will be.

Hansen solubility parameters for numerous solvents can be found in, among others, CRC Handbook of Solubility

Parameters and Other Cohesion Parameters, Second Edition by Allan F.M. Barton, CRC press 1991; Hansen Solubility

Parameters: A User's Handbook by Charles M. Hansen, CRC press 2007. It is also explained in these handbooks how analogous, equivalent solubility parameters have been derived by alternative methods to the original Hansen method, resulting in similarly useful parameters such as Hoy' s cohesion parameters for liquids .

It is preferred that the Hansen solubility parameter distance R _a with respect to acetic acid as determined at 25 °C is 12 MPa ^{1 2} or less, preferably 10 MPa ^{1 2} or less, more preferably 8 MPa ^{1 2} or less, most preferably 5 MPa ^{1 2} or less.

It was further found by the present inventors that selective recovery of acetic acid from aqueous streams is obtained when the 1-octanol/water partition coefficient of the absorbing solvent is relatively high. The 1- octanol/water partition coefficient, commonly expressed as its logarithmic value logPow^ represents the relative concentrations of a compound when dissolved in a mixture of 1-octanol and water at equilibrium, according to the following expression: lOgPow = "log [Coctanol/C _wa ter] (2) wherein

Coctanoi = concentration of the compound in 1-octanol

Coctanoi = concentration of the compound in water

As such, in the context of the present invention, the partition coefficient is a measure for the hydrophobicity of an absorbing solvent. Without wishing to be bound by theory, it is the inventors' belief that solvents having a suitably high partition coefficient are effective in minimizing the absorption of water from the acetic acid-water mixture.

Suitable absorbing solvents for use as described herein have a partition coefficient logP ₀ w as determined at 25 °C and pH 7 of at least 0. Typically, the absorbing solvent for use as described herein has a logP ₀ w of at least 0.5, preferably at least 1.0, more preferably at least 1.5, even more preferably at least 2.0, most preferably at least 3.0.

Experimentally determined 1-octanol/water partition coefficients for several organic solvent classes are listed in, for example, James Sangster, Octanol-Water Partition

Coefficients of Simple Organic Compounds, J. Phys . Chem. Ref. Data, Vol.18, No. 3, 1989. Where experimentally determined partition coefficients are not accessible, several

established reliable methods for calculating logP ₀ w values are available; these include the proprietary methods ClogP (Bio-Loom; BioByte Corp. /Pomona College) and miLogP

(Molinspiration Cheminformatics ) (see also Mannhold, M. et al. Calculation of Molecular Lipophilicity: State-of-the-Art and Comparison of Log P Methods on more than 96, 000

compounds. J. Pharm. Sci. 2009, 98, 861-893).

In order to realize cost-effective separation (recovery) of the absorbing solvent from acetic acid by e.g.

distillation, advantageously the absorbing solvent has a boiling point at atmospheric pressure that is at least 5 °C higher, preferably at least 10 °C higher, more preferably at least 20 °C higher than the boiling point of acetic acid.

Thus, for the recovery of acetic acid, which has a boiling point of 117 °C at atmospheric pressure, it is preferred that the absorbing solvent has a boiling point of at least 122 °C, or 125 °C. Preferably, it has a boiling point of at least 130 °C, more preferably at least 150 °C, even more preferably at least 170 °C.

From an economic perspective, it is preferred that the absorbing solvent has a boiling point that does not exceed 300 °C, more preferably not exceeds 280 °C, even more preferably not exceeds 250 °C, most preferably not exceeds 225 °C, at atmospheric pressure, in order to avoid excessive heating expenditure and eventual thermal degradation of the solvent .

Suitable oxygen-containing compounds having a Hansen solubility parameter distance R _a , partition coefficient and boiling point ranges as defined herein can be found in the classes of carboxylic acids, esters of carboxylic acids, ethers, aldehydes, ketones, alcohols and organic phosphates. These oxygen-containing component may be linear, branched or cyclic, saturated or unsaturated, and may be aliphatic or contain aromatic rings. Examples of such compounds include organic phosphates such as triethyl phosphate and tributyl phosphate, heterocyclic hydrocarbons such as benzofuran, carboxylic esters such as methyl benzoate, n-butyl butyrate, n-butyl acrylate, 2-ethylhexyl acetate, diethyl phthalate, isopropyl acetate, octyl acetate and cyclohexyl acetate, ketones such as acetophenone, dipropyl ketone and 5-ethyl-2- nonanone, high-boiling functionalized ethers such as anisole, diethylene glycol ethyl ether, diethylene glycol monobutyl ether, propylene glycol phenyl ether, 2-butoxy ethanol, 2- phenoxy ethanol and butyl diglycol acetate and, depending on the carboxylic acid to be recovered, higher-boiling

carboxylic acids such as pentanoic acid, hexanoic acid, heptanoic acid and octanoic acid. Based on the criteria as provided herein for the Hansen solubility parameter distance R _a , partition coefficient and boiling point, and taking into account the boiling point of the carboxylic acid to be recovered, the skilled person will be capable of selecting suitable absorbing solvents from each of these classes of oxygen-containing organic compounds .

Particularly preferred oxygen-containing compounds having a Hansen solubility parameter distance R _a , partition

coefficient and boiling point as defined herein are selected from the class of protic oxygenates, i.e. containing hydroxyl (-OH) group such as acids and alcohols and more preferably organic alcohols. Herein, organic alcohols are understood to organic compounds wherein one or more hydroxyl functional groups (-OH) are bound to a carbon atom. This includes linear, branched and cyclic alcohols, saturated and

unsaturated alcohols, primary, secondary or tertiary

alcohols, and aromatic as well as aliphatic alcohols. The alcohol may contain one hydroxyl group, or may contain two (diol) or more (triol, etc.) hydroxyl groups, provided that any surplus of hydroxyl groups does not result in an

undesirably high affinity for water. The alcohols for use according to the invention may further contain other

functional groups, such as oxygen-containing groups such as carbonyl, acid-, ether- or ester functional groups.

Preferred alcohols for use according to the invention are cyclic or aromatic alcohols having 6 to 20 carbon atoms, linear aliphatic alcohols having 6 to 14 carbon atoms and branched aliphatic alcohols having 5 to 14 carbon atoms.

In one asopect, the invention relates to process for the recovery of acetic acid from a vaporous aqueous stream, comprising

providing a vaporous aqueous stream comprising acetic acid,

contacting said aqueous stream comprising acetic acid with an absorbing solvent in an absorption unit, to produce a first stream comprising absorbing solvent and acetic acid and a second stream comprising water,

feeding said first stream comprising absorbing solvent and acetic acid to a solvent recovery unit, to produce a third stream comprising acetic acid and a fourth stream comprising absorbing solvent, and optionally recycling at least a portion of the fourth stream comprising absorbing solvent to the absorption unit ,

wherein the absorbing solvent is a cyclic or aromatic alcohol having 6 to 20 carbon atoms, a linear aliphatic alcohol having 6 to 14 carbon atoms or a branched aliphatic alcohol having 5 to 14 carbon atoms.

Examples of cyclic alcohols include unsubstituted and alkyl-substituted cyclohexanols and cyclopentanols, such as cyclohexanol, methyl cyclohexanol , methyl cyclopentanol, trimethyl cyclohexanols and (4-methylcyclohexyl) methanol; examples of aromatic alcohols include phenol, benzyl alcohol,

1- phenyl ethanol, 2-phenyl ethanol, cumyl alcohol (2-phenyl-

2-propanol) , xylenols (such as 2, 6-xylen-l-ol) , guaiacol (2- methoxyfenol ) , creosol, cresols such as m-cresol, phenoxy ethanol and naphthol; examples of suitable linear alcohols include those having the general formula C _n H _n+ iOH, wherein n is in the range of 6 to 14, preferably in the range of from 8 to 12, such as 1-hexanol, 2-hexanol, 3-hexanol, 1-octanol and 2-octanol, 1-decanol, 2-decanol, 1-dodecanol and 2-dodecanol; examples of suitable branched alcohols include those having in the range of 5 to 14, preferably in the range of 6 to 12 carbon atoms, such as 2-methyl-2-pentanol, 2-methyl-3- pentanol, 3-methyl-3-pentanol, 2-methyl-l-pentanol, 2,3- dimethyl-l-butanol , 2, 2-dimethyl-l-butanol, 2, 3-dimethyl-2- butanol, 3, 3-dimethyl-2-butanol, 4-methyl-l-pentanol (iso- hexanol) , 4-methyl-2-pentanol, 2-ethyl-l-butanol , 5-methyl-2- hexanol, 3-methyl-2-hexanol, 2, 2-dimethyl-l-pentanol, 4,4- dimethyl-l-pentanol, 2-ethyl-l-hexanol (iso-octanol ) , di- isobutyl carbinol (2, 6-dimetyl-4-heptanol) , 2-propyl

heptanol, 3-methyl-l-butanol (isopentyl alcohol) , 2-methyl-l- butanol, 2-benzyloxy-ethanol , 2-phenoxy ethanol and 2-butoxy- ethanol . Examples of alcohols containing other functional groups, such as oxygen-containing groups like aldehyde, ether- or ester groups, are diacetone alcohol and methyl salicylate. Other suitable alcohols include terpene-based alcohols such as pinacol, citronellol, menthol, and isoborneol.

Particularly preferred absorbing solvents for use according to the invention are 1-hexanol, 1-octanol, 1- decanol, 1-dodecanol, 2-ethyl-hexanol, diisobutyl carbinol, cresols, xylenols, anisole, butyl butyrate and 2-ethyl-hexyl- acetate .

An overview of suitable absorbing solvents for use according to the invention, including their Hansen solubility parameter distance R _a with respect to acetic acid, 1- octanol/water partition coefficient and boiling point is provided in Table 1.

Table 1. Values for Hansen solubility parameter distance Ra with respect to acetic acid at 25 °C, 1-octanol/water partition coefficient at 25 °C and pH 7, and boiling point at atmospheric pressure. Values for Hansen solubility parameter distance R _a have been calculated from the known values of 5 _d , δ _ρ , and 5 _h for acetic acid (5 _d = 14.5; δ _ρ =8.0; 5 _h = 13.5; all in MPa ^0'5 ) , and of the solvent using Equation (1) as provided above. Hansen solubility parameters are taken from CRC

Handbook of Solubility Parameters and Other Cohesion

Parameters, Second Edition by Allan F.M. Barton, CRC press 1991; Hansen Solubility Parameters: A User's Handbook by Charles M. Hansen, CRC press 2007. LogP ₀ w values are taken from James Sangster, Octanol-Water Partition Coefficients of Simple Organic Compounds, J. Phys . Chem. Ref. Data, Vol.18, No. 3, 1989, from technical data sheets supplied by solvent manufacturers or calculated using miLogP software

(Molinspiration Cheminformatics ) .

R _a bp

Solvent LogPow

(MPa ^{0 5} ) (°C)

acetic acid 0 -0.13 117

2-ethylbutanol 5 1.78 124

n-butyl acetate 9 1.77 125

2-methyl-3-pentanol 1 2.98 127

3-methyl-1-butanol 2 1.14 131

1.33

iso-pentyl alcohol 4 131

(calc)

n-butyl acrylate 9 2.39 145

2-butyl 1-octanol 7 5.05 145

5-methyl-2-hexanol 2 1.97 148

iso-hexanol (4-methyl-l-

4 1.6 152

pentanol )

anisole 10 2.11 153 cyclohexanone 11 0.81 155 cyclohexanol 7 1.32 161 furfural 14 0.41 162 n-butyl butyrate 10 2.83 165

2-butoxy-ethanol 4 0.8 171 cyclohexyl acetate 9 2.29 173 benzofuran 12 2.67 174 di-isobutyl carbinol 6 3.31 178

2-octanol 4 2.90 179 iso-octanol (2-ethyl

6 2.72 180 hexanol )

phenol 7 1.46 181 pentanoic acid 5 1.39 185

1-octanol 5 3.05 195 methyl benzoate 12 2.2 199

2-ethylhexyl acetate 10 3.71 200

2 , 6-xylenol 6 2.4 201 acetophenone 14 1.58 202 cresol (m) 8 1.94 203

3.84 octyl acetate 10 203

(calc) guaiacol 4 1.34 205 benzyl alcohol 8 1.1 205 hexanoic acid 4 1.84 206 triethyl phosphate 7 1.08 215 isophorone 7 2.07 215

2-propyl heptanol 3 4.4 218

3.62 iso-decanol 3 220

(calc)

4.2

1-decanol 9 220

(calc) isopropyl acetate 6 1.28 220 2-undecanol 4 4.4 229

3.32

octanoic acid 7 237

(calc)

butyl diglycol acetate 7 1.1 238

propylene glycol phenyl

7 1.41 241

ether

4.66

1-undecanol 3 243

(calc)

1-phenoxy ethanol 7 1.1 245

2-phenoxy ethanol 7 1.2 247

5.02

2-dodecanol 5 257

(calc)

1-dodecanol 4 5.13 259

1.17

2-benzyloxy ethanol 7 265

(calc)

tributyl phosphate 10 2.5 289

The oxygen-containing solvents as defined herein are characterized by having very good selectivity, as compared to water, for acetic acid. Furthermore, these solvents have relatively high boiling points and low volatility, thus minimizing their loss as vapour in the top stream of an absorption unit and allowing efficient separation from acetic acid as the bottom stream using in a subsequent distillation (solvent recovery) step.

It is possible to combine the absorbing solvent with one or more other solvents. In one embodiment, a mixture of two or more absorbing solvents as defined herein are used. In another embodiment, an absorbing solvent as defined herein is combined with one or more solvents selected from carboxylic esters, ethers, aldehydes or ketones. When one or more absorbing solvents as defined herein are used in admixture with another solvent not according to the invention, it is preferred that the one or more absorbing solvents with Hansen solubility parameter distance R _a , partition coefficient and boiling point as defined herein are present in a

concentration of at least 40 wt%, more preferably at least 50 wt%, even more preferably at least 70 wt%, most preferably at least 80 wt% or 90 wt% based on total weight of the solvent mixture. In one embodiment, the solvent mixture contains less than 40 wt%, preferably less than 30 wt%, more preferably less than 20 wt%, even more preferably less than 10 wt% of amine. In one embodiment, the one or more absorbing solvents as defined herein are used in the absence of amine compounds. In one embodiment, the absorbing solvent is employed in the absence of any other solvent not according to the invention. In order to avoid loss of solvent with acetic acid, it is further preferred that if a mixture of solvents is used, that such mixture contains less than 20 wt%, more preferably less than 10 wt%, even more preferably less than 5 wt%, most preferably less than 2 wt%, based on total weight of the solvent mixture, of a solvent having a boiling that is less than 5 °C higher than the boiling point of acetic acid.

In one embodiment, the solvent mixture may comprise one or more organic alcohols as defined herein and additionally one or more of the corresponding acetate esters, which may form during absorption and/or regeneration (desorption) of the absorbing alcohol solvent. If this is undesirable, these esters may at least partially be hydrolyzed, for example by feeding steam to the bottom of the column in the absorption or solvent regeneration (desorption) step.

The invention further relates to the use of an oxygen- containing organic compound as fully defined above as a solvent for absorbing acetic acid from a water-containing vapour stream. Depending on, among others, the concentration of acetic acid in the aqueous feed stream, the amount of absorbing solvent employed in the absorption process may vary within wide ranges, for example in a ratio (wt/wt) of absorbing solvent to acetic acid supplied to the absorption unit in the range of from 100:1 to 0.1:1, preferably in the range of from 50:1 to 0.25:1, more preferably in the range of from 40:1 to 0.5:1.

The amount of absorbing solvent fed to the absorption unit, depending on the feed rate of the aqueous stream comprising acetic acid, the concentration of acetic acid, and other process parameters such as temperature and pressure, is typically in the range of from 20:1 to 1:1, more preferably in the range of from 10:1 to 1:1, most preferably in the range of from 5:1 to 1:1.

The temperature in the absorption step may vary within wide ranges due to the selection of different mixtures of acid and solvents and operation pressures. It is within the ability of one skilled in the art to select appropriate operating temperature for a given mixture at a given

pressure .

Typically, the temperature in the absorption unit as described herein is in the range of of from 80 to 300 °C, more preferably 90 to 260 °C, most preferably 100 to 250 °C. The pressure in the absorption unit may also vary within wide ranges. Typically, the pressure in the absorption unit is in the range of of from 0.1 to 20 bar, more preferably 1 to 10 bar, most preferably 2 to 6 bar.

In one embodiment, the temperature is at most 50 °C, preferably at most 20 °C, more preferably at most 10 °C, most preferably at most 5 °C higher than the condensation

temperature of the acetic acid at operating pressure. In one embodiment, the temperature is at least 0 °C, preferably at least 10 °C, more preferably at least 20 °C, most preferably at least 30 °C above the condensation temperature of water at operating pressure.

In one embodiment, the pressure is at least 50 %, preferably at least 80 %, more preferably at least 100 %, most preferably at least 120 % of the condensation pressure of the acetic acid at operating temperature. Furthermore, the pressure is typically at most 99 %, preferably at most 90 %, more preferably at most 80 %, even more preferably at most 70 %, most preferably at most 50 % of the condensation pressure of water at operating temperature.

Advantageously, substantially all of the acetic acid present in the vaporous aqueous feed stream of the absorption unit exits said absorption unit in the absorbing solvent stream. Typically, at least 90 wt%, preferably at least 95 wt%, more preferably at least 99 wt%, even more preferably at least 99.5 wt%, yet even more preferably at least 99.8 wt%, most preferably at least 99.9 wt% of acetic acid present in the feed stream of the absorption unit is recovered in the absorbing solvent stream of said absorption unit.

Furthermore, in order to avoid the need for any further water removal steps, it is preferred that the absorbing solvent entrains substantially none of the water present in the gaseous aqueous feed stream of the absorption unit.

Preferably, the absorbing solvent effluent stream of the absorption unit comprises water and acetic acid in a weight ratio of less than 1:1, more preferably less than 0.5:1, even more preferably less than 0.1:1, yet even more preferably less than 0.05:1, most preferably less than 0.01:1 or about zero.

In the solvent recovery unit, acetic acid is removed (desorbed) from the absorbing solvent resulting in a product stream comprising acetic acid and another stream comprising the absorbing solvent now depleted of acetic acid.

In the solvent recovery unit, recovery of the absorbing solvent, and of optional other solvents present, is typically effectuated by distilling the effluent stream of the

absorption unit comprising acetic acid and absorbing solvent, resulting in a top stream comprising acetic acid and a bottom stream comprising the absorbing solvent. Distillation may be carried out in any distillation unit known to the skilled that is suitable for separating absorbing solvent from acetic acid, and it is within the ability of one skilled in the art to select appropriate operating conditions for obtaining a desired degree of product purity and/or solvent recovery. Typically, the temperature in the solvent recovery unit would vary depending on the solvent/mixture of solvents selected and is in the range of of from 80 to 300 °C, more preferably 100 to 250 °C, most preferably 110 to 200 °C. The pressure in the solvent recovery unit is suitably in the range of of from 0.1 to 10 bar, more preferably 0.5 to 5 bar, most preferably 1 to 3 bar.

In one embodiment, the temperature in the solvent recovery unit is at least 0 °C, preferably at least 10 °C, more preferably at least 20 °C, most preferably at least 30 °C above the condensation temperature of the carboxylic acid at operating pressure. In one embodiment, the temperature in the solvent recovery unit is at most 20 °C, preferably at most 10 °C, more preferably at most 5 °C, most preferably at most 0 °C below the condensation temperature of the absorbing solvent at operating pressure.

Typically, the pressure is at least at least 100 %, more preferably at least 110 %, even more preferably at least 120 %, most preferably at least 130 % of the condensation pressure of the absorbing solvent at operating temperature. Typically, the pressure is at most 100 %, preferably at most 90 %, more preferably at most 80 %, even more preferably at most 70 %, most preferably at most 50 % of the condensation pressure of acetic acid at operating temperature.

In one embodiment, steam is fed at the bottom of the solvent regeneration unit to hydrolyze any esters that may have been formed in the acetic acid/solvent mixture.

It is preferred that at least 80 wt%, more preferably at least 90 wt%, even more preferably at least 95 wt%, yet even more preferably at least 98 wt% of the acetic acid present in the stream fed to the solvent recovery unit comprising acetic acid and absorbing solvent is recovered.

It is further preferred that at least 80 wt%, more preferably at least 90 wt%, even more preferably at least 95 wt%, yet even more preferably at least 98 wt% of the solvent present in the stream fed to the solvent recovery unit comprising acetic acid and absorbing solvent is recovered.

Typically, the acetic acid product stream of the solvent recovery unit comprises acetic acid in a concentration of at least 70 wt%, preferably at least 80 wt%, more preferably at least 90 wt%, more preferably at least 95 wt%, even more preferably at least 99 wt%, yet even more preferably at least 99.5 wt%, most preferably at least 99.9 wt%.

Based on the amount of acetic acid present in the aqueous stream provided to the absorption unit, at least 50 wt%, more preferably at least 75 wt%, even more preferably at least 90 wt%, yet even more preferably at least 95 wt%, most

preferably at least 99 wt% of acetic acid is recovered in the process as defined herein.

In a preferred embodiment, at least a portion of the stream of the solvent recovery unit comprising the absorbing solvent, typically the bottom stream of a distillation unit, is recirculated to the absorption unit. Typically, at least 20 wt%, preferably at least 50 wt%, more preferably at least 70 wt%, most preferably at least 90 wt% of the recovered solvent stream is recirculated to the absorption unit. In one embodiment, the entire bottom stream comprising the absorbing solvent is recirculated to the absorption unit.

In the absorption column typically a top stream

comprising or substantially consisting of water vapour, and optionally other gases lighter than water, is produced. Water may be recovered from this top stream using a condensation step, for example by cooling down the top stream of the absorption unit to a lower temperature, for example room temperature, so that the water can be recovered as a liquid stream .

The water vapour top stream of the absorption unit may further comprise entrained absorbing solvent. Typically, said top stream of the absorption unit comprises no more than 3 vol%, preferably at most 1 vol%, more preferably at most 0.3, even more preferably at most 0.1, most preferably at most 0.01 vol% of entrained absorbing solvent. Said entrained absorbing solvent may be recovered by liquid-liquid

separation from the liquid water formed in the aforementioned condensation step. Advantageously, such liquid-liquid separation occurs spontaneously upon condensation due to the preferred poor miscibility of water and the absorbing solvent. In a preferred embodiment, the absorbing solvent thus recovered is at least partially recirculated to the absorption unit either as a separate stream or by mixing with a recirculated absorbing solvent stream from the solvent recovery unit .

The top stream comprising acetic acid originating from the solvent recovery unit may be further treated downstream, for example to further remove water by (azeotropic)

distillation, pervaporation, etc., and/or other purification methods available in the art to obtain the purity and specifications for acetic acid products according to market requirements .

Detailed description of the drawing

In Figure 1, a vapour stream 4 comprising water and acetic acid is fed to an absorption column 5 to which further an absorbing solvent 6 is fed. Acetic acid is absorbed by the absorbing solvent, which exits the absorption column as "fat" solvent stream 7. A vapour stream comprising water and other gaseous compounds exits the absorption column as stream 8.

Stream 7 comprising fat absorbing solvent and absorbed acetic acid is fed supplied to a solvent recovery

(desorption) unit, comprising a distillation unit 9 equipped with condenser section 9a and reboiler section 9b. Desorbed acetic acid leaves distillation unit 9 as stream 10, while absorbing solvent now depleted of absorbed acetic acid exits distillation unit 9 as stream 11. The acetic acid-depleted absorbing solvent stream 11 may be partially recirculated to absorption column 5 as absorbing solvent recirculation stream 12. Acetic acid stream 10 may be further purified downstream.

The vapour stream 8 comprising water and other gaseous compounds obtained as a top stream from absorption column 5 is fed to a condensation unit 13, where water is removed via stream 14. A product stream comprising gaseous compounds is removed via stream 15, from where it may undergo further separation and/or purification further downstream.

In condensation unit 13, spontaneous separation from the condensed water of absorbing solvent entrained in vapour stream 8 originating from absorption column 5 may occur. This separated absorbing solvent stream 16 may at least partially be recirculated to absorption column 5 via recirculation stream 17.

The invention is further illustrated by the following Examples .

EXAMPLES

General: Modeling of process thermodynamics

Modeling of the thermodynamics of the carboxylic acid separation and solvent recovery processes was performed using a Shell proprietary PSRK (Predictive Soave-Redlich-Kwong) - UNIFAC (UNIQUAC Functional-group Activity Coefficients) method which uses a modified Soave-Redlich-Kwong (SRK) cubic equation of state as the basis for both gas/vapour and liquid phases, connected to the UNIFAC group contribution activity coefficient model through an excess free energy mixing rule. The activity coefficient model is used to adapt the equation of state parameters and calculate the excess properties of vapour and liquid phases. The PSRK-UNIFAC framework is integrated in the Aspen Plus (AspenTech) process simulation software used for chemical process optimization, while parameter libraries are Shell proprietary.

Example 1. Vapour-phase absorption of acetic acid

A vapour-phase stream is provided having the following composition :

This stream is directly supplied to a counter-current absorption column in which the acetic acid is absorbed from the vapour phase by a liquid solvent consisting of 1-decanol. The temperature of the vaporous effluent supplied to the absorption column is 120 °C, the 1-decanol solvent is supplied at a temperature of 50 °C. The pressure of the vaporous effluent is 3.7 bar, the pressure of the 1-decanol solvent is 5.0 bar. The volume ratio of absorbing solvent to vaporous effluent is 1.74xl0 ^~3 .

The top vapour phase is condensed in a heat exchanger and sent to a vapour-liquid-liquid decanter. The vapour phase containing the ethylene product and other gases unconverted or co-produced in the oxidative dehydrogenation reactor are supplied to an ethylene purification unit. A high purity (about 99.9 wt%) water phase is obtained at the vapour- liquid-liquid decanter and sent to a water treatment unit and the small solvent phase is refluxed back to the absorption column .

At the bottom of the absorption column an acetic acid and solvent mixture ("fat solvent") is withdrawn, which is subsequently supplied to an solvent recovery column, wherein acetic acid is obtained at the top. An acetic acid-depleted ("lean solvent") stream is withdrawn at the bottom of the solvent recovery column, which is subsequently cooled and recycled to the absorption column. A minor solvent purge is performed in order to minimize solvent losses while avoiding potential build-up of impurities.

The composition of the various streams produced in this process is as follows:

Product streams

Stream Acetic acid Vaporous Aqueous Solven

[solvent [VLL [VLL t recovery] decanter decanter [purge

( t%) ] ] ]

( t %) ( t %) (wt %)

Water 37.86 2.07 99.83 0.00

Acetic acid 61.93 0.00 0.15 0.01

1-decanol 0.12 0.02 0.00 99.99

Other 0.09 97.90 0.02 0.00 components

99% of the acetic acid in the effluent is recovered as a concentrated (> 60 wt%) acetic acid stream.

Example 2: Vapour-phase absorption of acetic acid

Example 1 is repeated, with the distinction that the composition of the vaporous stream is as follows:

In this case, the volume ratio of solvent to vaporous effluent is 3.03xl0 ^~3 . The composition of the various streams produced in this absorption process is as follows: Product streams

Stream Acetic Vaporous Aqueous Solvent acid [VLL [VLL [purge]

[solvent decanter] decanter] (vol%) recovery (vol%) (vol%)

top]

(vol%)

Water 22.43 2.08 99.54 0.00

Acetic 77.39 0.01 0.44 0.01 acid

1-decanol 0.07 0.02 0.00 99.99

Other 0.12 97.90 0.02 0.00 components

99% of the acetic acid in the effluent of the ethane oxidative dehydrogenation process is recovered as a concentrated (> 75 wt%) acetic acid stream.